As a result of increased use of wireless communication systems, the demand for radio spectrum and spectrum-efficient systems is also increasing. One approach to easing the demand for spectrum is the so-called “secondary use” mode of operation where a radio device or system is allowed to use an unused or lightly used portion of spectrum otherwise restricted to primary users. Primary users are typically licensed by an appropriate authority to use one or more channels in a reserved spectrum region, if not an entire spectrum region. Examples of primary users include incumbent services such as television and commercial broadcast radio, as well as land mobile radio systems or public safety radio systems. Each of these systems, as well as other primary systems, are typically licensed operators and as such have an exclusive right to operate in a designated spectrum region or channel. However, some frequencies in these reserved spectrum bands may be unoccupied. For example, even in heavily populated metropolitan regions, there are typically unused television channels. These unused frequencies or channels may be made available for secondary use, provided the secondary users comply with certain regulations.
A secondary user must first check a candidate channel to determine if there is an incumbent on the channel, or substantially near (in frequency) the candidate channel. The incumbent may be a primary operator, or another secondary user. The incumbent may be transmitting directly on the channel, or in a spectrum substantially adjacent to the candidate channel or partially overlapping the candidate channel. The presence of a strong incumbent signal may eliminate the candidate channel from consideration for secondary use. However, if the strength of the incumbent signal is sufficiently weak, or the separation between the primary and secondary user is great enough, as may be indicated by a geo-location database, the secondary user may be able to use the candidate channel so long as there is sufficiently low interference with the incumbent signal being received at nearby receivers tuned to receive the incumbent signal.
Typically, the secondary user using sensing techniques for spectrum access has no recourse if an incumbent signal strength is at a threshold level, but not low enough to allow secondary transmission on the candidate channel. A secondary user may also reduce the power of its transmissions to avoid interfering with an incumbent. Accordingly, there is a need for a means by which a secondary user can reduce interference with an incumbent signal to allow or expand use of a candidate channel in the presence of an incumbent signal.